Some 1-phenyl-1,4-dihydro-4-oxopyridazines, variously substituted at position 3, are described in the literature. Among these compounds, the carboxylic acids exhibit a plant growth regulant activity and are used as gametocides (French patent application FR-A-2,383,605, Rohm and Haas Company). Other acids, variously substituted on the aromatic ring, and their esters, have an activity which stimulates the central nervous system (U.S. Pat. Nos. 2,792,396 and 2,835,671, Ciba Pharmaceutical Products). Compounds aminated at position 3, with an activity on the central nervous system, have already been described by the applicant (French patent application FR-A-2,648,135, Pierre Fabre Medicament) (published Sep. 27, 1991). See also corresponding EPA 0 402 227, published Dec. 12, 1990 and U.S. Pat. No. 5,059,599, issued Oct. 22, 1991. In addition, 1-phenyl-1,4-dihydro-3-hydroxy-4-oxopyridazine has been synthesized by Stahelin et al. (Helv. Chim. Acta 39 1741-1754, 1956), who do not mention any pharmacological activity.